In the early 1950's electronic computers were in their infancy. Los
Alamos National Laboratory acquired a MANIAC I, one of the first such
computers. The new tools made it possible to explore problems that
could not be solved with pencil and paper, leading to the creation of
a new subdiscipline: computational physics. This was the setting for
a landmark study in nonlinear mechanics by Enrico Fermi, J. Pasta,
Stanislaw Ulam, and Mary Tsingou (FPUT) [1]. (The article
does not list Mary Tsingou as a co-author, but acknowledges her
contribution, programming and operating the MANIAC I, a highly
nontrivial task for experts. By today's standards, she would most
likely have been accorded authorship status.)

The FPUT experiment was designed to investigate the process of
thermalization in a complex mechanical system. The system they chose
was a vibrating one-dimensional chain of masses connected with springs
that exerted a weakly anharmonic (nonlinear) restoring force. Thus the
number of degrees of freedom was finite. The anharmonic force law is
conservative (i.e. energy is conserved). The motion of a perfectly
harmonic (linear) chain can be described in terms of normal modes. If
such an harmonic chain is set into motion with only one normal mode
excited, it continues to vibrate in only that mode. Thus the energy
of motion is concentrated in one mode and remains so forever.
Anharmonic terms in the force law induce coupling among the normal
modes. So if a slightly anharmonic chain is set into motion in one of
the harmonic modes, one expects that with time, other modes will be
excited and the initial energy of motion will be redistributed among
them. The motion is quite complex and one might ask whether such a
system is complex enough that the laws of statistical mechanics apply,
namely, that with time the energy of motion will be distributed
uniformly among all the degrees of freedom -- i.e. normal modes.
They were surprised at what they found. You will be, too.